Android(安卓)Volley库源码简析（HTTP Request部分）
16lz
2021-12-04
本文仅对Volley中关于Http Request部分的一些简单用例做解析
源码目录树
首先，用脚本生成了该项目源码的目录树：
[android]┗━[volley] ┣━AuthFailureError.java ┣━Cache.java ┣━CacheDispatcher.java ┣━DefaultRetryPolicy.java ┣━ExecutorDelivery.java ┣━InternalUtils.java ┣━Network.java ┣━NetworkDispatcher.java ┣━NetworkError.java ┣━NetworkResponse.java ┣━NoConnectionError.java ┣━ParseError.java ┣━RedirectError.java ┣━Request.java ┣━RequestQueue.java ┣━Response.java ┣━ResponseDelivery.java ┣━RetryPolicy.java ┣━ServerError.java ┣━TimeoutError.java ┣━[toolbox] ┃ ┣━AndroidAuthenticator.java ┃ ┣━Authenticator.java ┃ ┣━BasicNetwork.java ┃ ┣━ByteArrayPool.java ┃ ┣━ClearCacheRequest.java ┃ ┣━DiskBasedCache.java ┃ ┣━HttpClientStack.java ┃ ┣━HttpHeaderParser.java ┃ ┣━HttpStack.java ┃ ┣━HurlStack.java ┃ ┣━ImageLoader.java ┃ ┣━ImageRequest.java ┃ ┣━JsonArrayRequest.java ┃ ┣━JsonObjectRequest.java ┃ ┣━JsonRequest.java ┃ ┣━NetworkImageView.java ┃ ┣━NoCache.java ┃ ┣━PoolingByteArrayOutputStream.java ┃ ┣━RequestFuture.java ┃ ┣━StringRequest.java ┃ ┗━Volley.java ┣━VolleyError.java ┗━VolleyLog.java可以看出，Volley源码放置得较为杂乱，不同功能模块的类并没有归到不同的包中。相比之下UIL的源码结构较为规范和合理。
从常用case入手，推断其项目架构
官网上给出的最简单的使用例子如下所示：
final TextView mTextView = (TextView) findViewById(R.id.text);...// 1. 新建一个QueueRequestQueue queue = Volley.newRequestQueue(this); String url ="http://www.google.com";// 2. 新建一个Request，写好listenerStringRequest stringRequest = new StringRequest(Request.Method.GET, url, new Response.Listener() { @Override public void onResponse(String response) { // Display the first 500 characters of the response string. mTextView.setText("Response is: "+ response.substring(0,500)); }}, new Response.ErrorListener() { @Override public void onErrorResponse(VolleyError error) { mTextView.setText("That didn't work!"); }});// 3. 将Request放到Queue里面执行queue.add(stringRequest); 结合下面这张图：
我们可以大致了解Volley的使用方法（见注释）和内部结构。下面就这个usecase展开进行源码级别的简述。
Volley类
Volley类提供了4个静态方法来方便用户新建Queue。其中：
public static RequestQueue newRequestQueue(Context context) { return newRequestQueue(context, null);}一句最终会调用：
// 传入 context，stack=null，maxDiskCacheBytes=-1public static RequestQueue newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) { File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR); String userAgent = "volley/0"; //1. 设置userAgent try { String packageName = context.getPackageName(); PackageInfo info = context.getPackageManager().getPackageInfo(packageName, 0); userAgent = packageName + "/" + info.versionCode; } catch (NameNotFoundException e) { } if (stack == null) { if (Build.VERSION.SDK_INT >= 9) { //2. 选择用哪个httpclient stack = new HurlStack(); } else { // Prior to Gingerbread, HttpUrlConnection was unreliable. // See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent)); } } Network network = new BasicNetwork(stack); RequestQueue queue; if (maxDiskCacheBytes <= -1) { // No maximum size specified queue = new RequestQueue(new DiskBasedCache(cacheDir), network); //3. 新建Queue } else { // Disk cache size specified queue = new RequestQueue(new DiskBasedCache(cacheDir, maxDiskCacheBytes), network); } queue.start();// 4. 传入Queue return queue;}值得注意的是：
Volley会根据SDK的version来决定使用java.net.HttpURLConnection（Build.VERSION.SDK_INT >= 9）还是org.apache.http.client.HttpClient
新建Queue后，Queue马上会被start。
stack类负责发送request（com.android.volley.Request）和获取response（org.apache.http.HttpResponse），network类负责分析和处理response，包装成NetworkResponse（com.android.volley.NetworkResponse）。
我们首先忽略掉network相关的细节，看一下queue的实现和request的调度策略。
RequestQueue
先来看一下RequestQueue的构造方法：
public RequestQueue(Cache cache, Network network) { this(cache, network, DEFAULT_NETWORK_THREAD_POOL_SIZE);}调用：
public RequestQueue(Cache cache, Network network, int threadPoolSize) { this(cache, network, threadPoolSize, new ExecutorDelivery(new Handler(Looper.getMainLooper())));}这里出现了一个新面孔ExecutorDelivery，根据字面意思可以猜测它是负责将请求的结果分发到主线程上，或者在主线程上执行回调（listener）。继续调用：
public RequestQueue(Cache cache, Network network, int threadPoolSize, ResponseDelivery delivery) { mCache = cache; mNetwork = network; mDispatchers = new NetworkDispatcher[threadPoolSize]; mDelivery = delivery;}这里又出现了一个新面孔NetworkDispatcher。留意到threadPoolSize这个数组长度参数的字面意义，结合上面的Volley架构图，猜想NetworkDispatcher是一个work thread，循环等待并通过network执行在Queue上的request。
RequestQueue被实例化后，便调用其start()方法：
public void start() { stop(); // Make sure any currently running dispatchers are stopped. // Create the cache dispatcher and start it. mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery); mCacheDispatcher.start(); // Create network dispatchers (and corresponding threads) up to the pool size. for (int i = 0; i < mDispatchers.length; i++) { NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork, mCache, mDelivery); mDispatchers[i] = networkDispatcher; networkDispatcher.start(); }}相应地有：
public void stop() { if (mCacheDispatcher != null) { mCacheDispatcher.quit(); } for (int i = 0; i < mDispatchers.length; i++) { if (mDispatchers[i] != null) { mDispatchers[i].quit(); } }}这里的逻辑很简单：
开始之前停止所有旧的任务（即interrupt所有worker thread）。
启动一个负责cache的worker thread。
启动n个负责network的worker thread。
worker thread开始不断地等待来自Queue的request。
Request
接下来执行queue.add(stringRequest); ，一个request被加入到queue中，代码如下所示：
public Request add(Request request) { // Tag the request as belonging to this queue and add it to the set of current requests. request.setRequestQueue(this); synchronized (mCurrentRequests) { mCurrentRequests.add(request); } // Process requests in the order they are added. request.setSequence(getSequenceNumber()); request.addMarker("add-to-queue"); // marker用来指示request当前的状态，实际上是用来打log // If the request is uncacheable, skip the cache queue and go straight to the network. if (!request.shouldCache()) { mNetworkQueue.add(request); return request; } // Insert request into stage if there's already a request with the same cache key in flight. synchronized (mWaitingRequests) { String cacheKey = request.getCacheKey(); if (mWaitingRequests.containsKey(cacheKey)) { // There is already a request in flight. Queue up. Queue> stagedRequests = mWaitingRequests.get(cacheKey); if (stagedRequests == null) { stagedRequests = new LinkedList>(); } stagedRequests.add(request); mWaitingRequests.put(cacheKey, stagedRequests); if (VolleyLog.DEBUG) { VolleyLog.v("Request for cacheKey=%s is in flight, putting on hold.", cacheKey); } } else { // Insert 'null' queue for this cacheKey, indicating there is now a request in // flight. mWaitingRequests.put(cacheKey, null); mCacheQueue.add(request); } return request; }} 这里的逻辑是：
对新加进来的request进行一些设置。
如果不需要cache，那么把request直接加到network queue中。
根据key检查request是否正在执行。如果是，则将其放入到waiting链表中。猜想当request完成的时候会调用某个方法将key在waiting链表中删除，然后依次执行waiting的request。如果否，则将其加入cache queue中。
CacheDispatcher
假设该uri访问是第一次执行，那么对应的request会被放到cache queue中。cache worker thread（cache dispatcher）发现cache queue中存在request，会马上将其dequeue并执行。我们来看一下CacheDispatcher的run方法：
public class CacheDispatcher extends Thread { ... private final Cache mCache; // 一开始传入了“new DiskBasedCache(cacheDir)” ... public void quit() { mQuit = true; interrupt(); } @Override public void run() { if (DEBUG) VolleyLog.v("start new dispatcher"); Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); // Make a blocking call to initialize the cache. mCache.initialize(); Request<?> request; while (true) { // release previous request object to avoid leaking request object when mQueue is drained. request = null; //确保最后一个request做完后能及时回收内存。 try { // Take a request from the queue. request = mCacheQueue.take(); // 堵塞 } catch (InterruptedException e) { // We may have been interrupted because it was time to quit. if (mQuit) { return; // 退出点 } continue; } try { request.addMarker("cache-queue-take"); // If the request has been canceled, don't bother dispatching it. if (request.isCanceled()) { request.finish("cache-discard-canceled"); continue; } // miss cache则直接将request放到network queue中 Cache.Entry entry = mCache.get(request.getCacheKey()); if (entry == null) { request.addMarker("cache-miss"); // Cache miss; send off to the network dispatcher. mNetworkQueue.put(request); continue; } // cache 过期了，直接将request放到network queue中 if (entry.isExpired()) { request.addMarker("cache-hit-expired"); request.setCacheEntry(entry); mNetworkQueue.put(request); continue; } // 将cache中的data包装成一个response request.addMarker("cache-hit"); Response<?> response = request.parseNetworkResponse( new NetworkResponse(entry.data, entry.responseHeaders)); request.addMarker("cache-hit-parsed"); if (!entry.refreshNeeded()) { // cache不需要刷新，直接将response交给delivery mDelivery.postResponse(request, response); } else { // cache需要刷新。现将旧的内容返回，同时将request放进network queue。 request.addMarker("cache-hit-refresh-needed"); request.setCacheEntry(entry); // Mark the response as intermediate. response.intermediate = true; // Post the intermediate response back to the user and have // the delivery then forward the request along to the network. final Request<?> finalRequest = request; mDelivery.postResponse(request, response, new Runnable() { @Override public void run() { try { mNetworkQueue.put(finalRequest); } catch (InterruptedException e) { // Not much we can do about this. } } }); } } catch (Exception e) { VolleyLog.e(e, "Unhandled exception %s", e.toString()); } } }}接下来看一下mDelivery.postResponse这个方法。
ExecutorDelivery
从上文得知，mDelivery是一个ExecutorDelivery的实例（在新建RequestQueue时传入）。
ExecutorDelivery的初始化代码如下所示：
public ExecutorDelivery(final Handler handler) { // Make an Executor that just wraps the handler. mResponsePoster = new Executor() { // java.util.concurrent.Executor; @Override public void execute(Runnable command) { handler.post(command); } };}关于java.util.concurrent.Executor可以看这篇文章，这里就不展开了。
postResponse代码如下所示：
@Overridepublic void postResponse(Request<?> request, Response<?> response, Runnable runnable) { request.markDelivered(); //标记为已分发 request.addMarker("post-response"); mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable)); // 在初始化时传入的handler中执行ResponseDeliveryRunnable}ResponseDeliveryRunnable是ExecutorDelivery的一个子类，负责根据request的不同结果调用对应的listener方法：
@SuppressWarnings("rawtypes")private class ResponseDeliveryRunnable implements Runnable { private final Request mRequest; private final Response mResponse; private final Runnable mRunnable; public ResponseDeliveryRunnable(Request request, Response response, Runnable runnable) { mRequest = request; mResponse = response; mRunnable = runnable; } @SuppressWarnings("unchecked") @Override public void run() { // 在主线程中执行 // If this request has canceled, finish it and don't deliver. if (mRequest.isCanceled()) { mRequest.finish("canceled-at-delivery"); // 会调用 RequestQueue的finish方法 return; } // Deliver a normal response or error, depending. if (mResponse.isSuccess()) { mRequest.deliverResponse(mResponse.result); //调用 listener的onResponse(response) } else { mRequest.deliverError(mResponse.error); } // If this is an intermediate response, add a marker, otherwise we're done // and the request can be finished. if (mResponse.intermediate) { mRequest.addMarker("intermediate-response"); } else { mRequest.finish("done"); } // If we have been provided a post-delivery runnable, run it. if (mRunnable != null) { mRunnable.run(); } }}接下来我们回头看看NetworkDispatcher对network queue的处理。
NetworkDispatcher
NetworkDispatcher的源码如下所示：
public class NetworkDispatcher extends Thread { private final Network mNetwork; // BasicNetwork实例 ... private final BlockingQueue> mQueue; // network queue ... public void quit() { mQuit = true; interrupt(); } @TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH) private void addTrafficStatsTag(Request<?> request) { // 方便统计Volley的网络流量 ... } @Override public void run() { Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); Request<?> request; while (true) { long startTimeMs = SystemClock.elapsedRealtime(); // release previous request object to avoid leaking request object when mQueue is drained. request = null; try { //1. 堵塞读取network queue中的request request = mQueue.take(); } catch (InterruptedException e) { // We may have been interrupted because it was time to quit. if (mQuit) { return; } continue; } try { request.addMarker("network-queue-take"); // If the request was cancelled already, do not perform the // network request. if (request.isCanceled()) { request.finish("network-discard-cancelled"); continue; } addTrafficStatsTag(request); //2. 在network对象中堵塞执行request NetworkResponse networkResponse = mNetwork.performRequest(request); request.addMarker("network-http-complete"); // If the server returned 304 AND we delivered a response already, // we're done -- don't deliver a second identical response. if (networkResponse.notModified && request.hasHadResponseDelivered()) { // 304表示资源未被修改 request.finish("not-modified"); continue; } //3. 将NetworkResponse转成Response Response<?> response = request.parseNetworkResponse(networkResponse); request.addMarker("network-parse-complete"); // Write to cache if applicable. // TODO: Only update cache metadata instead of entire record for 304s. if (request.shouldCache() && response.cacheEntry != null) { // 4. Response放到cache中 mCache.put(request.getCacheKey(), response.cacheEntry); request.addMarker("network-cache-written"); } //5. 通过Delivery回调结果 request.markDelivered(); mDelivery.postResponse(request, response); } catch (VolleyError volleyError) { volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs); parseAndDeliverNetworkError(request, volleyError); } catch (Exception e) { VolleyLog.e(e, "Unhandled exception %s", e.toString()); VolleyError volleyError = new VolleyError(e); volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs); mDelivery.postError(request, volleyError); } } } private void parseAndDeliverNetworkError(Request<?> request, VolleyError error) { error = request.parseNetworkError(error); mDelivery.postError(request, error); }} NetworkDispatcher的处理流程和CacheDispatcher差不多，见注释。TrafficStats的介绍可以看这里。
上述代码的关键在于mNetwork.performRequest(request)和request.parseNetworkResponse(networkResponse)这两个调用。
Network
Network是一个接口，只有一个performRequest(Request<?> request)方法：
public interface Network { public NetworkResponse performRequest(Request<?> request) throws VolleyError;}本文例子中Network的实现类是BasicNetwork：
public class BasicNetwork implements Network { protected static final boolean DEBUG = VolleyLog.DEBUG; private static int SLOW_REQUEST_THRESHOLD_MS = 3000; private static int DEFAULT_POOL_SIZE = 4096; protected final HttpStack mHttpStack; protected final ByteArrayPool mPool; public BasicNetwork(HttpStack httpStack) { // If a pool isn't passed in, then build a small default pool that will give us a lot of // benefit and not use too much memory. this(httpStack, new ByteArrayPool(DEFAULT_POOL_SIZE)); } ...}注意到BasicNetwork的两个关键的成员：mHttpStack和mPool，和对apache依赖：
import org.apache.http.Header;import org.apache.http.HttpEntity;import org.apache.http.HttpResponse;import org.apache.http.HttpStatus;import org.apache.http.StatusLine;可我们先来看一下performRequest()的执行流程：
public NetworkResponse performRequest(Request<?> request) throws VolleyError { long requestStart = SystemClock.elapsedRealtime(); while (true) { // 依赖 org.apache.http.HttpResponse HttpResponse httpResponse = null; byte[] responseContents = null; Map responseHeaders = Collections.emptyMap(); try { // 1. 生成header Map headers = new HashMap(); addCacheHeaders(headers, request.getCacheEntry()); // 2. 通过httpstack发起请求。注意‘发起请求’这个动作不在request中进行，request只是保存着请求的信息。 httpResponse = mHttpStack.performRequest(request, headers); // 3. 获得请求结果的一些信息 StatusLine statusLine = httpResponse.getStatusLine(); int statusCode = statusLine.getStatusCode(); responseHeaders = convertHeaders(httpResponse.getAllHeaders()); // 4. 通过statusCode(304)来判断是否可以直接使用cache if (statusCode == HttpStatus.SC_NOT_MODIFIED) { Entry entry = request.getCacheEntry(); if (entry == null) { return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, null, responseHeaders, true, SystemClock.elapsedRealtime() - requestStart); } // 从cache中取出data，返回新的NetworkResponse entry.responseHeaders.putAll(responseHeaders); return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, entry.data, entry.responseHeaders, true, SystemClock.elapsedRealtime() - requestStart); } // 5. 通过statusCode通过判断是否需要重定向 if (statusCode == HttpStatus.SC_MOVED_PERMANENTLY || statusCode == HttpStatus.SC_MOVED_TEMPORARILY) { String newUrl = responseHeaders.get("Location"); request.setRedirectUrl(newUrl); } // 6. 取出reponse中的data，为字节数组 // Some responses such as 204s do not have content. We must check. if (httpResponse.getEntity() != null) { // 通过entityToBytes从outputstream中读取数据，throws IOException responseContents = entityToBytes(httpResponse.getEntity()); } else { // Add 0 byte response as a way of honestly representing a // no-content request. responseContents = new byte[0]; } // if the request is slow, log it. long requestLifetime = SystemClock.elapsedRealtime() - requestStart; logSlowRequests(requestLifetime, request, responseContents, statusLine); if (statusCode < 200 || statusCode > 299) { throw new IOException(); } return new NetworkResponse(statusCode, responseContents, responseHeaders, false, SystemClock.elapsedRealtime() - requestStart); } catch (SocketTimeoutException e) { attemptRetryOnException("socket", request, new TimeoutError()); } catch (ConnectTimeoutException e) { attemptRetryOnException("connection", request, new TimeoutError()); } catch (MalformedURLException e) { throw new RuntimeException("Bad URL " + request.getUrl(), e); } catch (IOException e) { // 7. 如果entityToBytes方法throw了IOException int statusCode = 0; NetworkResponse networkResponse = null; if (httpResponse != null) { statusCode = httpResponse.getStatusLine().getStatusCode(); } else { throw new NoConnectionError(e); } if (statusCode == HttpStatus.SC_MOVED_PERMANENTLY || statusCode == HttpStatus.SC_MOVED_TEMPORARILY) { VolleyLog.e("Request at %s has been redirected to %s", request.getOriginUrl(), request.getUrl()); } else { VolleyLog.e("Unexpected response code %d for %s", statusCode, request.getUrl()); } // 如果reponseContent有数据 if (responseContents != null) { networkResponse = new NetworkResponse(statusCode, responseContents, responseHeaders, false, SystemClock.elapsedRealtime() - requestStart); // 那么根据statusCode执行重试 if (statusCode == HttpStatus.SC_UNAUTHORIZED || statusCode == HttpStatus.SC_FORBIDDEN) { attemptRetryOnException("auth", request, new AuthFailureError(networkResponse)); } else if (statusCode == HttpStatus.SC_MOVED_PERMANENTLY || statusCode == HttpStatus.SC_MOVED_TEMPORARILY) { attemptRetryOnException("redirect", request, new RedirectError(networkResponse)); } else { // TODO: Only throw ServerError for 5xx status codes. throw new ServerError(networkResponse); } } else { throw new NetworkError(e); } } }} attemptRetryOnException()代码如下所示：
private static void attemptRetryOnException(String logPrefix, Request<?> request, VolleyError exception) throws VolleyError { RetryPolicy retryPolicy = request.getRetryPolicy(); int oldTimeout = request.getTimeoutMs(); try { // 关键语句 retryPolicy.retry(exception); } catch (VolleyError e) { request.addMarker( String.format("%s-timeout-giveup [timeout=%s]", logPrefix, oldTimeout)); throw e; } request.addMarker(String.format("%s-retry [timeout=%s]", logPrefix, oldTimeout));}RetryPolicy是一个接口：
public interface RetryPolicy { public int getCurrentTimeout(); public int getCurrentRetryCount(); public void retry(VolleyError error) throws VolleyError;}如没有特殊指定，request中的RetryPolicy为DefaultRetryPolicy，其retry方法实现如下：
public void retry(VolleyError error) throws VolleyError { mCurrentRetryCount++; mCurrentTimeoutMs += (mCurrentTimeoutMs * mBackoffMultiplier); if (!hasAttemptRemaining()) { throw error; }}如果还没超出重试次数上限，那么不会抛出异常，并返回到performRequest()的while循环中。接下来分析一下BaseNetwork的entityToBytes()方法：
private byte[] entityToBytes(HttpEntity entity) throws IOException, ServerError { // 1. 新建PoolingByteArrayOutputStream PoolingByteArrayOutputStream bytes = new PoolingByteArrayOutputStream(mPool, (int) entity.getContentLength()); byte[] buffer = null; try { InputStream in = entity.getContent(); if (in == null) { throw new ServerError(); } // 2. 在字节池中取出1024字节buffer buffer = mPool.getBuf(1024); int count; // 3. 从entity的inputStream中读出数据到buffer while ((count = in.read(buffer)) != -1) { // 将buffer写到PoolingByteArrayOutputStream中 bytes.write(buffer, 0, count); } // 4. 将所有数据返回 return bytes.toByteArray(); } finally { try { // Close the InputStream and release the resources by "consuming the content". entity.consumeContent(); } catch (IOException e) { // This can happen if there was an exception above that left the entity in // an invalid state. VolleyLog.v("Error occured when calling consumingContent"); } // 5. 归还buffer到字节池 mPool.returnBuf(buffer); bytes.close(); }}执行步骤见代码注释。这里不对ByteArrayPool类和PoolingByteArrayOutputStream展开。
HttpStack
HttpStack是一个接口，仅负责将request发送出去：
public interface HttpStack { public HttpResponse performRequest(Request<?> request, Map additionalHeaders) throws IOException, AuthFailureError;} 从最开始的对Volley类的分析可知，SDK version > 9的情况下使用HurlStack(java.net.HttpURLConnection)，否则使用HttpClientStack(org.apache.http.client.HttpClient)。
stack各自实现了performRequest()方法，在内部正式发起了http请求。具体的用法参考各自的api文档，这里不详细展开了。
Request
Request类主要是保存着该次请求的参数和该次请求当前的状态，本身不含有请求相关的行为：
public abstract class Request implements Comparable> { ... public interface Method { int DEPRECATED_GET_OR_POST = -1; int GET = 0; int POST = 1; int PUT = 2; int DELETE = 3; int HEAD = 4; int OPTIONS = 5; int TRACE = 6; int PATCH = 7; } ... private final int mMethod; private final String mUrl; private String mRedirectUrl; private String mIdentifier; private final int mDefaultTrafficStatsTag; private Response.ErrorListener mErrorListener; private Integer mSequence; private RequestQueue mRequestQueue; private boolean mShouldCache = true; private boolean mCanceled = false; private boolean mResponseDelivered = false; private RetryPolicy mRetryPolicy; ...} 下面再来分析一下request.parseNetworkResponse(networkResponse)这个方法。以StringRequest为例：
@Overrideprotected Response parseNetworkResponse(NetworkResponse response) { String parsed; try { parsed = new String(response.data, HttpHeaderParser.parseCharset(response.headers)); } catch (UnsupportedEncodingException e) { parsed = new String(response.data); } return Response.success(parsed, HttpHeaderParser.parseCacheHeaders(response));} 可以看到它只是简单地将data转换成string，然后返回一个success的response。
而JsonObjectRequest的实现如下：
@Overrideprotected Response parseNetworkResponse(NetworkResponse response) { try { String jsonString = new String(response.data, HttpHeaderParser.parseCharset(response.headers, PROTOCOL_CHARSET)); return Response.success(new JSONObject(jsonString), HttpHeaderParser.parseCacheHeaders(response)); } catch (UnsupportedEncodingException e) { return Response.error(new ParseError(e)); } catch (JSONException je) { return Response.error(new ParseError(je)); }} 它现将data转换成string，然后再生成一个JSONObject返回。
总结
综上，Volley的大致框架如下所述：
一个RequestQueue中包含两个内部queue，分别是cache queue和network queue。还有一个cache dispatcher和n个network dispatcher，它们都继承成于Thread，分别负责执行缓存和网络请求。还有一个delivery，负责分发请求结果。
cache dispatcher在独立的线程上运行。cache dispatcher循环等待、取出并执行cache queue中的request。把结果交给delivery。
N个network dispatcher分别在独立的线程上运行。network dispatcher循环等待、取出并执行network queue中的request。把结果交给delivery和添加到cache中。
delivery负责在主线程上将结果传给相应的listener回调。
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